Field
The present invention relates to techniques for improving the aerodynamic profile of crafts with multi-faceted dihedral angle planform design, and in particular, for improving crafts with flat surfaces for low observability using insufflation.
Background
Stealth craft design tends to adhere to fundamental shaping principles. For example, the Lockheed F-22 aircraft is shaped with leading and trailing edges of the wing and tail having identical sweep angles (a design technique called planform alignment). The fuselage and canopy have sloping sides. The canopy seam, bay doors, and other surface interfaces are saw-toothed. The vertical tails are canted. The engine face is deeply hidden by a serpentine inlet duct and weapons are carried internally.
Despite the benefits of shape design to achieve low observability to scanners, such as radar (or sonar, depending on the craft), there are often additional factors that can enhance observability. These include engines, fuel, avionics packages, electrical and hydraulic circuits, and people.
Radar absorbing materials, in the case of stealth craft, serve to reduce an aircraft radar cross section (RCS) against specific threats, and to isolate multiple antennas on the aircraft to prevent cross talk.
There are two basic approaches to passive RDS reduction: (i) shaping to minimize backscatter, and (ii) coating for energy absorption and cancellation. Both of these approaches have to be used coherently in aircraft design to achieve the required low observable levels over the appropriate frequency range in the electromagnetic spectrum.
There is a tremendous advantage to positioning surfaces so that the radar wave strikes them at close to tangential angles and far from right angles to edges. To a first approximation, when the diameter of a sphere is significantly larger than the radar wavelength, its radar cross section is equal to its geometric frontal area. The return of a one-square-meter sphere is compared to that from a one-meter-square plate at different look angles. One case to consider is a rotation of the plate from normal incidence to a shallow angle, with the radar beam at right angles to a pair of edges. The other is with the radar beam at 45 degrees to the edges. The frequency is selected so that the wavelength is about 1/10 of the length of the plate, in this case very typical of acquisition radars on surface to air missile systems. At normal incidence, the flat plate acts like a mirror, and its return is 30 decibels (dB) above (or 1,000 times) the return from the sphere. If we now rotate the plate about one edge so that the edge is always normal to the incoming wave, we find that the cross section drops by a factor of 1,000, equal to that of the sphere, when the look angle reaches 30 degrees off normal to the plate. As the angle is increased, the locus of maxima falls by about another factor of 50, for a total change of 50,000 from the normal look angle. Now if you go back to the normal incidence case and rotate the plate about a diagonal relative to the incoming wave, there is a remarkable difference. In this case, the cross section drops by 30 dB when the plate is only eight degrees off normal, and drops another 40 dB by the time the plate is at a shallow angle to the incoming radar beam. This is a total change in radar cross section of 10,000,000!
From this, it would seem that it is fairly easy to decrease the radar cross section substantially by merely avoiding obviously high-return shapes and attitude angles.
Shaping requirements have strong negative influence on an aircraft's aerodynamic properties. For this reason, an aircraft such as the F-117 has poor aerodynamics, is inherently unstable, and cannot be flown without computer assistance.
Also, shaping does not offer stealth advantages against low-frequency radar. If the radar wavelength is roughly twice the size of the target, a half-wave resonance effect can still generate a significant return. However, low-frequency radar is limited by lack of available frequencies which are heavily used by other systems, lack of accuracy given the long wavelength, and by the radar's size, making it difficult to transport.
Acoustic stealth plays a primary role in submarine stealth as well as for ground vehicles, but also in aircraft.
In addition to reducing infrared and acoustic emissions, a stealth vehicle must avoid radiating any other detectable energy, such as from onboard radars, communications systems, or RF leakage from electronics enclosures.
The size of a target's image on radar is measured by the radar cross section. Modern stealth aircraft are said to have an RCS comparable with small birds or large insects, though this varies widely depending on aircraft and radar. Because RCS is directly related to a target's cross-sectional area, the only way to reduce it is to make the physical profile smaller. When reflecting much of the radiation away or absorbing it altogether, a stealth object achieves a smaller radar cross section.
To reflect radiation without the use of high-return shapes and attitude angles, dihedral angle planform design may be utilized. Unfortunately, sharp dihedral-edge design has not been consistently implemented, most importantly due to poor aerodynamics associated with sharp dihedral angles.
Half-true dihedrals have been adopted that achieve stealth benefits and at same time address the need to shape the craft to accommodate people and components, such as on-board equipment and fuel. Half-true dihedrals are characterized by slightly rounded dihedral- edge areas which help to reduce cavitation (e.g., wear, resistance to air, and/or fuel consumption) and/or air ionization (trailability).
Half-true dihedrals tradeoff better aerodynamics and the need to accommodate people and equipment for lower observability (better stealth capability).
It is desirable to be able to provide an improved stealth design configuration which addresses the problems of conventional systems.